This invention relates to hollow fiber membrane fluid separation modules which are specially adapted for boreside feed of a mixture of fluid to be separated.
Hollow fiber membrane modules are commonly divided into two or three regions, wherein such regions are sealed so that fluid cannot communicate from one region to the other, except by passing the fluid through the bores of the hollow fibers or permeating the fluid across the walls of the hollow fibers. Generally, a hollow fiber membrane module comprises a bundle of hollow fibers arranged in a fashion such that each end of the hollow fibers are embedded in a resin matrix commonly referred to as a tubesheet or header. Such hollow fibers communicate through the tubesheets and are open on the opposite face of each tubesheet. The opposite face of the tubesheet means herein that face of the tubesheet which is opposite the bundle. Generally, the regions of a membrane separation module are divided by the tubesheets and seals about the tubesheets. In a shellside feed membrane module, the fluids to be separated are introduced into the module in the region between the tubesheets and the outside of the fibers, and the fluids which permeate through the hollow fiber membranes into the bores of the hollow fibers are removed at one or both ends of the hollow fibers in the regions adjacent to the opposite face of one or both tubesheets. The non-permeating fluids are removed from a region in the area between the tubesheets but outside of the fibers. Most commercial industrial fluid separation processes operate in this fashion.
In a shellside feed process, the region about the hollow fiber bundle is pressurized. In a bore side feed process, the mixture of fluids to be separated is introduced into one end of the membrane module adjacent to the opposite face of one of the tubesheets such that the mixture will flow down the bores of the hollow fibers through the tubesheet and into the region between the tubesheets. In the region between the tubesheets, the fluid which selectively permeates through the membrane is removed from the shellside of the membrane. Those fluids which do not permeate through the membrane exit into a region adjacent to the opposite face of the second tubesheet and are removed from that region. In this operation the pressure then is exerted on the faces of the tubesheets which are opposite the fiber bundle. The hollow fibers are also pressurized in this operation. As the tubesheets are usually comprised of a resinous material, significant bending, compressive, and sheer stresses are exerted on such tubesheets by this operation. This creates a problem with supporting the tubesheets and preventing them from collapsing in on the hollow fiber bundle.
A second problem in boreside feed is getting proper flow of the permeate on the shellside of the fibers. One of the driving forces in a membrane separation process is a concentration gradient across the membrane. As the mixture of fluids to be separated flows down the bores and the more selectively permeable fluids permeate through the hollow fibers, the concentration of the selectively permeable fluids along the hollow fibers is reduced and the concentration on the shellside of the selectively permeable fluids increases. This results in a lowering of the driving force. A third problem is that if the flow on the shellside of the membranes is not properly controlled, it will have areas of high concentration of the permeate fluid and flow to the exit ports may not occur in an efficient manner.
What is needed is a hollow fiber membrane fluid separation module which is adapted for boreside feed. What is further needed is such a membrane module which minimizes the stresses on the tubesheets. What is further needed is such a membrane module in which the flow of the permeate on the shellside is controlled to maximize the concentration gradients along the fibers, to prevent areas of high permeate concentration, and to enhance the flow of permeate on the shellside.